In a printing system (also referred to herein as a “printer”) or a copier system (also referred to herein as a “copier”), an image input module is used to measure reflection from an image bearing surface and from test patches on the image bearing surface. Often, these image input modules are referred to as densitometers, as they are imaging the image bearing surface to detect the toner deposition or lack thereof on the image bearing surface. These measured reflections are used for calibration of the printer or copier.
In prior systems, the image input module used illumination apparatus comprising a fluorescent lamp or a rare gas lamp for illuminating substantially linear portions of the image bearing surface and the test patches. However, fluorescent lamps and rare gas lamps are relatively expensive. More recently, systems have employed illumination apparatus comprising discrete, light emitting diodes (LEDs) which are located on a conventional printed circuit board. In such illumination systems, the LEDs perform nearly as point sources which together operate to provide illumination along substantially linear portions of the image bearing surfaces.